ReconnectingWebSocket drops messages the server sends right after the handshake #1141

Found in review of #1128.

Problem

ReconnectingWebSocket.connect() (src/websocket/reconnectingWebSocket.ts) attaches its message listener to the underlying socket only after await this.#socketFactory() resolves. The factory, createOneWayWebSocket in src/api/coderApi.ts, resolves through waitForOpen on the open event.

The server can send its first frame in the same chunk as the handshake. ws emits it on a process.nextTick, which runs before the awaited promise resumes connect(). So that message reaches no listener, and subscribers never see it. The stream logger attached in attachStreamLogger is registered before open, so it still logs the message; only delivery is lost.

This happens on the first connect and on every reconnect.

Impact

Any stream whose server writes right after the handshake can lose its first message. For example, watchAgentMetadata can show nothing until the agent's next metadata update. I haven't checked each stream: the workspace watch, build logs and agent logs may lose their first event or backlog the same way. The SSE fallback (SseConnection) isn't checked either.

Repro

A plain ws 8.22.0 client against a server that sends one message on connection. The client resolves a promise on open, awaits it, then adds its message listener, mirroring connect():

listener after awaited open: 20/20 dropped
listener before open:         0/20 dropped

import { WebSocket, WebSocketServer } from "ws";

const server = new WebSocketServer({ port: 0 });
server.on("connection", (socket) => socket.send("initial"));
await new Promise((resolve) => server.once("listening", resolve));
const url = `ws://127.0.0.1:${server.address().port}`;

async function attempt(listenFirst) {
	const socket = new WebSocket(url);
	let received = false;
	const onMessage = () => (received = true);
	if (listenFirst) socket.on("message", onMessage);
	await new Promise((resolve, reject) => {
		socket.once("open", resolve);
		socket.once("error", reject);
	});
	if (!listenFirst) socket.on("message", onMessage);
	await new Promise((resolve) => setTimeout(resolve, 100));
	socket.terminate();
	return received;
}

for (const listenFirst of [false, true]) {
	let dropped = 0;
	for (let i = 0; i < 20; i++) if (!(await attempt(listenFirst))) dropped++;
	console.log(`${listenFirst ? "before" : "after awaited"} open: ${dropped}/20 dropped`);
}
server.close();

Fix

Attach the message (and other) listeners before the socket opens. For example, have the factory return the socket unopened and let connect() wait for open after registering its listeners, or have OneWayWebSocket buffer messages until a listener is attached. Add a test where the server sends a message as soon as the connection opens.